Single error channel monopulse system

ABSTRACT

This is a monopulse system for deriving the azimuth and elevation error signals in a single channel, eliminating the second error channel used in the conventional monopulse systems and reducing the number of channels required for the sum azimuth and elevation error signals from three to two channels. This reduction from three to two channels is accomplished by introducing a mode generator in the throat of the feed horn. The mode generator selectively changes the phase of signals in certain modes while leaving the phases of other modes uneffected. These selected signals are processed through a circular polarizer which introduces a time phase delay to the signals unaffected by the mode generator so that in the single error channel two signals appear, the azimuth and the elevation error signals, with the elevation error delayed 90* in relation to the azimuth error. These signals are in phase quadrature, can be processed by basic phase comparison techniques and the azimuth and elevation and error information can be separately extracted.

Elite Patent 191 Grabowski et al.

[ 1 Jan. 30, 1973 [75] Inventors: Joseph P. Grabowski, Willingboro;Walter E. Powell, Jr., Cinnaminson, both of NJ.

[73] Assignee: The United States of America as represented by theSecretary of the Navy [22] Filed: April 19, 1971 [21] Appl. No.: 134,969

UNITED STATES PATENTS Miller .,343/l6 M X Foldes ..343/l6 M X PrimaryExaminerT. H. Tubbesing An0rneyR. S. Sciascia, O. E. Hodges and J. I.Rosenblatt [57] ABSTRACT This is a monopulse system for deriving theazimuth and elevation error signals in a single channel,

eliminating the second error channel used in the conventional monopulsesystems and reducing the number of channels required for the sum azimuthand elevation error signals from three to two channels. This reductionfrom three to two channels is accomplished by introducing a modegenerator in the throat of the feed horn. The mode generator selectivelychanges the phase of signals in certain modes while leaving the phasesof other modes uneffected. These selected signals are processed througha circular polarizer which introduces a time phase delay to the signalsunaffected by the mode generator so that in the single error channel twosignals appear, the azimuth and the elevation error signals, with theelevation error delayed 90 in relation to the azimuth error. Thesesignals are in phase quadrature, can be processed by basic phasecomparison techniques and the azimuth and elevation and errorinformation can be separately extracted.

22 Claims, 5 Drawing Figures 8 1 e l ,I (I I3 MAGIC MODE MULTI- POWERCIRCULAR TS-5, DIVIDER POLARIZER g'fig gk 1:122

/59 t t 57 55 t Q 53 SUM i t 0 PORT 0 PHASE 0 PHASE LSEH LSE" DIFFERENCEMODE PORT HAS AZERR-O SINGLE ERROR CHANNEL MONOPULSE SYSTEM DESCRIPTIONOF THE PRIOR ART Monopulse radar systems using four and five portexciters are well known in the art, however, these monopulse systemspresently used required at least two channels to generate the azimuthand the elevation error signals; a channel is required for each of theerror signals.

SUMMARY OF THE INVENTION This invention pertains to the field ofmonopulse antenna systems and particularly pertains to a multimode feedsystem producing monopulse tracking patterns for two orthogonal trackingplanes while using a single tracking channel for azimuth and elevationsignals.

With the understanding that reciprocity exists between the transmittingand the receiving mode of this system, the invention is summarized firstwith respect to a transmitting mode.

Illumination patterns are generated for sum, azimuth error and elevationerror by the appropriate combination of modes in the mode generator andphaser section of the feed system. If vertical polarization isintroduced into the four symmetrical square waveguide inputs of the modegenerator such that the upper pair of square waveguides is fed 180 outof phase with the lower pairs, the field configuration at the input tothe mode generator is equivalent to the sum of equal amplitudes of theTE and TM modes. In the mode generator section, the relative phasebetween the TE mode and the TM mode can be changed 180 by means ofspecially configured step ridges centered on each wall of the modegenerator. The output of the mode generator feed horn will then be ahorizontally polarized azimuth E plane error pattern.

A horizontally polarized TE mode, H plane elevation error pattern can begenerated, if horizontal polarization is introduced at the foursymmetrical waveguide inputs such that the upper and lower pairs of thewaveguide inputs are 180 out of phase. The mode generator has no effecton the'TE mode so that this mode passes through the mode generatorunchanged.

Referring now to a received signal, it is seen that an azimuth E planeerror signal appearing at the feed horn is rotated 90 by the modegenerator so that the output of the mode generator entering the fourport exciter is a vertically polarized E plane azimuth errorpattern.This vertical azimuth E plane error pattern enters the polarizer andpasses through the polarizer with zero degrees time phase delay comparedto horizontal E fields entering the polarizer. A power divider combinesthe upper and lower pairs of the square waveguide sections into a singleupper and lower waveguide connected to a magic tee hybrid. The errorsignal is then developed at the difference port of the magic tee hybridand the sum signal portion of the received signal is developed at thesum port of the magic tee hybrid.

The elevation error signal (TE mode) enters the feed horn and passesthrough the mode generator unchanged by the mode generator. Thehorizontal E fields then enter the polarizer where vertical E fieldcomponents are generated at the polarizer output. These vertical Efields have been delayed 90 in time phase compared to the vertical Efields from the azimuth error signal. A power divider combines the upperand lower pairs of square guide sections into a single upper and lowerwaveguide connected to a magic tee hybrid. Since the vertical E fieldsof the elevation error signal have been delayed in time phase comparedto the vertical E fields from the azimuth error signal return, two errorsignals from two orthogonal tracking planes appear at the differenceport of the magic tee hybrid and these signals appear in phasequadrature. These phase quadrature signals can then be processed bybasic phase comparison circuits, used in monopulse tracking systems, sothe azimuth and elevation errorsignalscan be extracted from a singlechannel while the sum signal is extracted from the sum port of the magictee hybrid.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. la shows a section view of themode generator multimode feed horn, and four port exciter of FIG. 1.

'FIG. 1 shows in perspective and partly exposed the multimode feed hornfour port exciter and the mode generator.

FIG. 2 is a block diagram of the system showing the generation of theazimuth error signal at the difference 1 port of the magic-tee hybrid.

FIG. 3 is a block diagram of the system showing elevation error signalgeneration at the difference port of the magic tee hybrid.

FIG. 4 shows the sum pattern (TE LSE modes) generated by the system andis included to orient the reader to the system.

DESCRIPTION OF THE PREFERRED EMBODIMENT Reference is now made to FIG.la, wherein is shown in a sectional view, the four port exciter, modegenerator and multimode feed horn sections of the monopulse systemcomprising the four port exciter 5 (exciter ports 1 and 3 shown), themode generator section 11, and the multimode feed horn 13. As shown inthe view of FIG. 1a, the mode generator section 11, comprises a plurali'ty of step ridges, step ridges 15 and 17 being shown. The. direction ofpropagation of the transmitted and radiated signal designated by arrow19 is from the four port exciters past the mode generator and spacersection 11 and through .feed horn 13 to the target.

A received signal would be received at feed horn l3 and is propagatedpast the mode generator section 11 to the four port exciter 5.

Referring now to FIG. 1, the mode generator section and the four portexciter is shown in perspective and partially exposed. As shown in FIG.1, the mode generator section comprises step ridges 14, 15, 16 and 17,these step ridge's being located on walls 21, 23, 25 and 27 of the modegenerator section 11. The plane of each step ridge is perpendicular tothe plane of its respective wall. In addition, each step ridge isaligned with an interior waveguide wall separating two ports of the fourport exciter (ridge l4 aligned with wall 29, ridge 15 aligned with wall31, ridge 16 aligned with wall 33, and ridge [7 aligned with wall 35).

The step ridges shown in FIG. 1, are used to generate the properproportion of the LSE mode (FIG. 4) to shape the'sum pattern (TE LSE forhigh aperture efficiency and to cause a relative phase variation betweenthe TE and the TM 1 modes thereby rotating the E plane error pattern(LSE mode) 90.

Although there is no precise way to define the configuration of the stepridges, the step ridges may be optimized by a trial and error approachwell known in the art. The considerations for the approximate sizes ofthe ridge dimensions are as follows:

a. The height 37 determines primarily the proportion of the LSE modegenerated and should be approximately 0.3x;

b. The length 39, effects the proportion of the LSE mode generated andprimarily the relative phase between the TE and the TM modes and shouldbe approximately I A;

c. The thinness 41, effects the TE mode, thin ridges are used tominimize any change in the TE mode and should .be approximately 1/16 A;

d. The gap 43, between the step ridge and the four port exciter effectsthe phasing and generation of the LSE mode and should be approximately1/1 6 )t;

e. The taper angle of the ridges, defined by dimension 45, 39 and 37,aids in optimizing the relative phasing of the TE, and TM, modes; and

f. The dimension of each of the walls 21 and 25, of the mode generatorsection should be approximately 1.3 A.

In FIG. 2, the system is shown in a block diagram. The azimuth errorsignal E field patterns are shown at various stages within the system.

The sum pattern (TE LSE is shown in FIG. 4, to orient the reader to thesystems transmitted and received signals. During transmission a sumpattern (TE mode) is generated through the four port exciter. The heightand length of each of the step ridges 14-17 effects the proportion ofthe LSE mode generated. The LSE mode generated in the mode generatorsection combines with the sum TE, pattern to produce the resultant sumpattern (TE LSE as shown in FIG. 4.

When a return signal is received from a target off axis in the azimuthplane, the E fields generated in the feed horn l3 produce an amplitudeunbalance across the horn aperature in the azimuth plane. The returnsignal generates the sum pattern (TE LSE and the E plane error pattern(LSE mode) as shown in block 51 of FIG. 2. The mode generator section11, causes a rotation of the LSIEZ mode such that its E fields arevertical by changing the relative phase between its TE and TM modes by180, as shown in block 51 and block 53 of FIG. 2. Four vertical E fieldsare then generated in the four port exciter 5, as shown in block 55.These four vertical E fields enter the circular polarizer 6, and passthrough the polarizer with zero degree time phase delay as shown inblock 57, compared to the horizontal E fields entering the polarizer.The power divider 7, combines the upper and lower pairs of squarewaveguide section connected to the polarizer output, into single upperand a single lower waveguides which feed the magic tee hybrid 8. Theerror signal, the difference between the upper and lower waveguidesignals feeding the magic tee hybrid is developed at the differencechannel port of the magic tee hybrid. The sum signal is developed at thesum channel port of the magic tee hybrid as in conventional monopulseradar systems.

Referring now to FIG. 3, the process for deriving the elevation errorsignal is shown. In FIG. 3, the block diagram is the same as that shownin FIG. 2, with the same numbers representing the same and similarlyoperating parts. The sum pattern (TE, mode) is generated in the fourport exciter 5. The length 39 and height 37 of the step ridges affectsthe proportion of LSE mode generated. The LSE mode generated in the modegenerator combines with the TE mode to produce the resultant sum pattern(TE LSE as shown in FIG, 4, which is transmitted through the horn 13 andradiated into space.

When a return signal is received from a target off axis in the elevationplane the E field generated in the horn l3 introduces an amplitudeunbalance in the vertical plane of the horn aperature. This imbalancecomprises the sum pattern mode (TE LSE and the H plane error patternmode TE The mode generator has no effect on the TE mode, the step ridgesbeing thin (less than l/l6 A so as not to effect the TE mode. The TEmode then propagates unchanged through the mode generator 11, to thefour port exciter 5, as shown in blocks 61 and 63 of FIG. 3. The fourhorizontal E fields at output of the four port exciter 5 enter thecircular polarizer 6, where vertical E fields components are generatedat the polarizer output. These vertical E fields have been delayed intime phase compared to the vertical E fields from the azimuth errorsignal return, shown in block 57. The power divider 7, combines theupper and lower pairs of square guide section into a single upper and asingle lower waveguide as shown in blocks 69, which feed the magic teehybrid. The elevation error signal is developed at the differencechannel port of the magic tee hybrid 8 and the sum signal is developedat the sum channel port.

We see therefore, that two error signals are developed at the singledifference channel port of the magic tee hybrid: an azimuth error signaland an elevation error signal. However, these signals are in time phasequadrature as the vertical E fields generated by the elevation errorsignal are delayed 90 in time phase compared to the vertical E fieldsfrom the azimuth error signal. These signals being in phase quadraturecan be processed by basic phase comparison techniques to separatelydetect the azimuth and elevation error signals.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed is:

l. A single error channel monopulse system comprisa multimode feed hornfor transmitting monopulse signals and for receiving monopulse E planeerror signal patterns;

a mode generator connected to said feed horn for receiving said E planeerror signal and for rotating by 90 degrees the E plane error patternmode signal received from a target off axis in the azimuth plane;

said mode generator receiving an elevation E plane error pattern fromsaid feed horn and passing said elevation E plane error pattern modeunchanged through said generator;

means connected to said mode generator for generating vertical E fieldcomponents from said elevation E plane error pattern mode;

said vertical E field components being in time phase quadrature relativeto said rotated azimuth E plane error pattern; and means connected tosaid generating means for detecting said azimuth and elevation errorsignals to determine the target azimuth and elevation error. 2. Thesystem of claim 1, wherein said detecting means includes a means toderive the monopulse sum, elevation, and azimuth error signals;

said deriving means having a single error channel for deriving said timephase quadrature related azimuth and elevation error signals. 3. Thesystem of claim 2, wherein: said deriving means is a magic tee hybridhaving input terminals and first and second output terminals; said sumsignal being developed at said first output terminal and said azimuthand elevation phase quadrature related signals being developed at saidsecond output terminal. 4. The system of claim 3, wherein: said rotatingmeans comprises a waveguide; a four port exciter; said waveguide beingconnected at one end to said multimode feed horn and being connected atits other end to said four port exciter; said waveguide having means onthe interior of each of the four walls of said waveguide for modegeneration and phase variation of transmitted and received monopulsesignals. 5. The system of claim 4, wherein: said means is a step ridgemounted on each of said waveguide walls. 6. The system of claim 5wherein: each wall of said four port exciter being coextensive with itsconnecting waveguide wall; said four port exciter having intersectinginterior walls defining the said four ports. 7. The system of claim 4,wherein: said means for mode generation and phase variation isresponsive to the sum TE pattern mode, generates a LSE mode to shape thesaid sum pattern transmitted from said multimode feed horn; said meansfor mode generation and phase variation responsive to an azimuth E planeerror pattern mode causing a time phase variation between the TE and TMmodes of the azimuth E plane error pattern in the LSE mode and rotatingsaid LSE E field pattern by 90. 8. The system of claim 7, wherein: saidmeans for mode generation is a step ridge mounted on each of saidwaveguide walls; the amount of said LSE mode generated is functionallyrelated to the height of each said step ridge and the length of eachsaid step ridge; said phase difference between said TM and TM mode isfunctionally related to each said step ridge length; the effect of saidstep ridges on the elevation error E plane pattern in the TE mode beingfunctionally related to the thinness of each said step ridge relative towave length of the TE, mode signal; said step ridges being spaced apartfrom said four port interior walls by a gap; the phase of said LSE modebeing functionally related to said gap size; and

the relative phasing between said TM and TE modes being functionallyrelated to the taper angle of each said step ridges.

9. The system of claim 4, wherein said means to generate vertical Efields components comprises circular polarizers;

' each said circular polarizer having an input connected to a respectiveport of said four port exciter for receiving the error signal from saidrespective port;

a power'divider connected to the output of said circular polarizers forcombining the signals from the upper pair of said waveguides of saidfour port exciter into a single signal and for combining the signalsfrom the lower pair of waveguides of said four port exciter into asingle second signal;

said magic tee hybrid being connected to receive said first and secondsignals for developing the sum signal at said sum port and fordeveloping said azimuth and elevation error signals at said differenceport;

said means for mode generation and phase variation responsive to anazimuth E plane error pattern mode causing a time phase variationbetween the TE and TM modes of the azimuth E plane error in the LSEpattern mode and rotating said LSE E fields by 10. The system of claim3, wherein:

said means to generate vertical E field components from said elevationerror signal comprises circular polarizers.

l 1. A mode generator, comprising a feed horn;

a waveguide;

a four port exciter;

said waveguide being connected at one end to said feed horn and beingconnected at said other end to said four port exciter; and

said waveguide having step ridges on each of its four walls for modegeneration and phase variation of transmitted and received monopulsesignals.

12. The system of claim 1 1, wherein:

a step ridge is mounted on each of said waveguide walls;.

13. The system of claim 12, wherein: each said four port exciter wall iscoextensive with its abutting waveguide wall;

said four port waveguide having intersecting interior walls for definingsaid four ports;-and

each said step ridge being mounted in the plane of an intersectingwaveguide wall of said four port exciter.

14. The system of claim 1 1, wherein:

said step ridges, responsive to the sum TE pattern mode generates a LSEmode to shape the sum pattern transmitted from I said multimode feedhorn;

said step ridges responsive to an azimuth E plane error pattern modecausing a time phase variation between the TE and TM modes of theazimuth E plane error in the LSE patternmode and rotating said LSE Efields by 90.

15. The system of claim 14, wherein:

the proportion of said LSE mode generated is functionally related to theheight of said step ridge and to the length of said step ridge;

said phase difference between said TM and TM modes is functionallyrelated to said step ridge length;

the effect of said step ridges on the elevation error E plane pattern inthe TB mode being functionally related to the thinness of said stepridge, relative to the wave length of the TE mode signal;

said step ridges being spaced apart from the interior walls of said fourport exciter by a gap;

the phase of said LSE mode being functionally related to said gap; and

the relative phasing between said TM and TE modes being functionallyrelated to the taper angle of said step ridges.

16. A method for generating azimuth and elevation error signals in asingle error channel, comprising the steps of:

rotating the azimuth E plane error signal pattern received from a targetoff axis in the azimuth plane by 90;

passing the elevation E plane error signal pattern through a polarizerto generate vertical E field components for said elevation error signaldelayed by 90 in time phase compared to said rotated azimuth errorpattern.

17. The method of claim 16, including the steps of:

detecting the azimuth error signal from said rotated azimuth errorpattern and detecting the elevation error signal from said elevationerror signal delayed by 90 in time phase relative to said rotatedazimuth pattern in a single error channel; and

producing separate azimuth and elevation error signals from said phasequadrature related azimuth and elevation error signal.

18. A single error channel monopulse system, comprising:

means for receiving a plurality of error pattern signals;

means for changing the phase of one of said plurality of error patternsignals, with respect to the other signals of said plurality of errorpattern signals, and producing phase related signals; and

means for detecting said phase related error pattern signals.

19. The system of claim 18, wherein:

said means for detecting said phase related error signals is limited toa single channel for producing said plurality of phase related errorpattern signals; and

including means to separate and extract each of said plurality of phaserelated error pattern signals, connected to said single channel.

20. The system of claim 19, wherein:

said means for changing in phase includes means for rotating one of saiderror pattern signals with respect to said other of said plurality oferror pattern signals; and

said one error pattern signal being changed in phase,

being a non-rotated error pattern signal.

21. The system of claim 20, wherein:

said error pattern signals are the E plane error patterns in the azimuthand elevation planes.

22. The system of claim 21, wherein:

said means for rotating rotates said azimuth signal.

1. A single error channel monopulse system comprising: a multimode feedhorn for transmitting monopulse signals and for receiving monopulse Eplane error signal patterns; a mode generator connected to said feedhorn for receiving said E plane error signal and for rotating by 90degrees the E plane error pattern mode signal received from a target offaxis in the azimuth plane; said mode generator receiving an elevation Eplane error pattern from said feed horn and passing said elevation Eplane error pattern mode unchanged through said generator; meansconnected to said mode generator for generating vertical E fieldcomponents from said elevation E plane error pattern mode; said verticalE field components being in time phase quadrature relative to saidrotated azimuth E plane error pattern; and means connected to saidgenerating means for detecting said azimuth and elevation error signalsto determine the target azimuth and elevation error.
 1. A single errorchannel monopulse system comprising: a multimode feed horn fortransmitting monopulse signals and for receiving monopulse E plane errorsignal patterns; a mode generator connected to said feed horn forreceiving said E plane error signal and for rotating by 90 degrees the Eplane error pattern mode signal received from a target off axis in theazimuth plane; said mode generator receiving an elevation E plane errorpattern from said feed horn and passing said elevation E plane errorpattern mode unchanged through said generator; means connected to saidmode generator for generating vertical E field components from saidelevation E plane error pattern mode; said vertical E field componentsbeing in time phase quadrature relative to said rotated azimuth E planeerror pattern; and means connected to said generating means fordetecting said azimuth and elevation error signals to determine thetarget azimuth and elevation error.
 2. The system of claim 1, whereinsaid detecting means includes a means to derive the monopulse sum,elevation, and azimuth error signals; said deriving means having asingle error channel for deriving said time phase quadrature relatedazimuth and elevation error signals.
 3. The system of claim 2, wherein:said deriving means is a magic tee hybrid having input terminals andfirst and second output terminals; said sum signal being developed atsaid first output terminal and said azimuth aNd elevation phasequadrature related signals being developed at said second outputterminal.
 4. The system of claim 3, wherein: said rotating meanscomprises a waveguide; a four port exciter; said waveguide beingconnected at one end to said multimode feed horn and being connected atits other end to said four port exciter; said waveguide having means onthe interior of each of the four walls of said waveguide for modegeneration and phase variation of transmitted and received monopulsesignals.
 5. The system of claim 4, wherein: said means is a step ridgemounted on each of said waveguide walls.
 6. The system of claim 5wherein: each wall of said four port exciter being coextensive with itsconnecting waveguide wall; said four port exciter having intersectinginterior walls defining the said four ports.
 7. The system of claim 4,wherein: said means for mode generation and phase variation isresponsive to the sum TE10 pattern mode, generates a LSE12 mode to shapethe said sum pattern transmitted from said multimode feed horn; saidmeans for mode generation and phase variation responsive to an azimuth Eplane error pattern mode causing a time phase variation between the TE11and TM11 modes of the azimuth E plane error pattern in the LSE11 modeand rotating said LSE11 E field pattern by 90*.
 8. The system of claim7, wherein: said means for mode generation is a step ridge mounted oneach of said waveguide walls; the amount of said LSE12 mode generated isfunctionally related to the height of each said step ridge and thelength of each said step ridge; said phase difference between said TM11and TM12 mode is functionally related to each said step ridge length;the effect of said step ridges on the elevation error E plane pattern inthe TE20 mode being functionally related to the thinness of each saidstep ridge relative to wave length of the TE20 mode signal; said stepridges being spaced apart from said four port interior walls by a gap;the phase of said LSE11 mode being functionally related to said gapsize; and the relative phasing between said TM11 and TE11 modes beingfunctionally related to the taper angle of each said step ridges.
 9. Thesystem of claim 4, wherein said means to generate vertical E fieldscomponents comprises circular polarizers; each said circular polarizerhaving an input connected to a respective port of said four port exciterfor receiving the error signal from said respective port; a powerdivider connected to the output of said circular polarizers forcombining the signals from the upper pair of said waveguides of saidfour port exciter into a single signal and for combining the signalsfrom the lower pair of waveguides of said four port exciter into asingle second signal; said magic tee hybrid being connected to receivesaid first and second signals for developing the sum signal at said sumport and for developing said azimuth and elevation error signals at saiddifference port; said means for mode generation and phase variationresponsive to an azimuth E plane error pattern mode causing a time phasevariation between the TE11 and TM11 modes of the azimuth E plane errorin the LSE11 pattern mode and rotating said LSE11 E fields by 90*. 10.The system of claim 3, wherein: said means to generate vertical E fieldcomponents from said elevation error signal comprises circularpolarizers.
 11. A mode generator, comprising a feed horn; a waveguide; afour port exciter; said waveguide being connected at one end to saidfeed horn and being connected at said other end to said four portexciter; and said waveguide having step ridges on each of its four wallsfor mode generation and phase variation of transmitted and receivedmonopulse signals.
 12. The system of claim 11, wherein: a step ridge ismounted on each of said waveguide walls;.
 13. The system of claim 12,wherein: each said four port exciter wall is coextensive with itsabutting waveguide wall; said four port waveguide having intersectinginterior walls for defining said four ports; and each said step ridgebeing mounted in the plane of an intersecting waveguide wall of saidfour port exciter.
 14. The system of claim 11, wherein: said stepridges, responsive to the sum TE11 pattern mode generates a LSE12 modeto shape the sum pattern transmitted from said multimode feed horn; saidstep ridges responsive to an azimuth E plane error pattern mode causinga time phase variation between the TE11 and TM11 modes of the azimuth Eplane error in the LSE11 pattern mode and rotating said LSE11 E fieldsby 90*.
 15. The system of claim 14, wherein: the proportion of saidLSE12 mode generated is functionally related to the height of said stepridge and to the length of said step ridge; said phase differencebetween said TM11 and TM12 modes is functionally related to said stepridge length; the effect of said step ridges on the elevation error Eplane pattern in the TE20 mode being functionally related to thethinness of said step ridge, relative to the wave length of the TE20mode signal; said step ridges being spaced apart from the interior wallsof said four port exciter by a gap; the phase of said LSE11 mode beingfunctionally related to said gap; and the relative phasing between saidTM11 and TE11 modes being functionally related to the taper angle ofsaid step ridges.
 16. A method for generating azimuth and elevationerror signals in a single error channel, comprising the steps of:rotating the azimuth E plane error signal pattern received from a targetoff axis in the azimuth plane by 90*; passing the elevation E planeerror signal pattern through a polarizer to generate vertical E fieldcomponents for said elevation error signal delayed by 90* in time phasecompared to said rotated azimuth error pattern.
 17. The method of claim16, including the steps of: detecting the azimuth error signal from saidrotated azimuth error pattern and detecting the elevation error signalfrom said elevation error signal delayed by 90* in time phase relativeto said rotated azimuth pattern in a single error channel; and producingseparate azimuth and elevation error signals from said phase quadraturerelated azimuth and elevation error signal.
 18. A single error channelmonopulse system, comprising: means for receiving a plurality of errorpattern signals; means for changing the phase of one of said pluralityof error pattern signals, with respect to the other signals of saidplurality of error pattern signals, and producing phase related signals;and means for detecting said phase related error pattern signals. 19.The system of claim 18, wherein: said means for detecting said phaserelated error signals is limited to a single channel for producing saidplurality of phase related error pattern signals; and including means toseparate and extract each of said plurality of phase related errorpattern signals, connected to said single channel.
 20. The system ofclaim 19, wherein: said means for changing in phase includes means forrotating one of said error pattern signals with respect to said other ofsaid plurality of error pattern signals; and said one error patternsignal being changed in phase, being a non-rotated error pattern signal.21. The system of claim 20, wherein: said error pattern signals are theE plane error patterns in the azimuth and elevation planes.